Here are my suggestions! Even though Avery is a top 20 name, I believe that it would compliment Emery's name quite well because they are both unisex names and have the same amount of syllables, without sounding too identical! Avery Ivena sounds like a beautiful combination! If Avery is too popular for you, a less popular (but still as beautiful name) would be Ever/Everly! Ever/Everly and Emery sounds like an amazing sib-set, and Ever/Everly Ivena is a stunning name combination! But if you think Ever/Everly is too unusual, a name that is a suitable middle (not too unusual but not too popular, this name ranks at #164, so it's out of the top 100, let alone the top 20) would be Eden. This name is a very sweet, girly, and if you are religious (biblical, referring to the Garden of Eden here). I feel with the middle name Ivena, it sounds just as pretty as Avery Ivena and Ever Ivena! With this name, you won't have to be concerned about having five other Eden's in your daughter's class (which would occur with Avery) but it's not as off the grid as Ever/Everly (if you're concerned about her name standing out too much), so it's a suitable middle name (as I previously mentioned) I hope that I helped you out and I wish you all the best in naming Emery's little sister!

Take care!

P.S.Here are the three names with the middle name Ivena below, so you can see how the names look together!